Power From the Heart of a Mountain
“THE day trip you’ll remember for a lifetime.” That is how my visit to the southwest of New Zealand’s South Island was described in the travel brochures. And it was true. The journey from Manapouri to Doubtful Sound, across water and mountains, brought me into contact with unique sights and impressive engineering achievements. It was like witnessing an eighth wonder of the world—a hydropower station buried in the heart of a mountain.
My journey also gave me a reminder of New Zealand’s oldest inhabitants, the Maori, and their ancient legends and languages. According to two Maori accounts, “Manapouri” may mean either ‘lake of sorrow or tears’ or ‘lake of the sorrowful heart.’ For me it was also the name of the town that was my starting point on that memorable day.
An Unexpected Sight
As our boat sped smoothly across the calm lake, magnificent U-shaped valleys and towering mountains came into view. We were fortunate to have a beautiful clear day, as this region gets up to 300 inches [7,500 mm] of rain a year! So it was a photographer’s paradise, with trees and exuberant vegetation growing from the waterline straight up the mountainsides. The noise of our boat was the only evidence of any human penetration into the region on our 75-minute journey across the lake. But a journey to what?
To an unexpected sight—at West Arm, at the far end of the lake, in the middle of nowhere, rose the switchyard of a hydroelectric power station. What could have impelled anyone to build a power station here, so far from human habitation? Only a unique set of geographic and geologic circumstances could have given any engineer or surveyor this inspiration.
That idea came in 1904 when P. J. Hay, a surveyor, noted the potential of this body of water. Its surface is over 600 feet [180 m] above sea level, and having a depth of nearly 1,500 feet [450 m], its bottom is about 850 feet [260 m] below sea level! Yet, it is separated from the sea by only about six miles [10 km] of mountainous terrain. But it would be another 60 years before his idea could be made a reality. What triggered the initiative? An Australian smelting corporation operating in New Zealand needed power for its smelter at Tiwai Point, near Invercargill, about a hundred miles [160 km] away as the crow flies. But how was electricity going to be generated?
Vision Made Reality
The plan, conceived by U.S.-based Bechtel engineering company, was to tunnel deep into the mountain called Leaning Peak and build a generating station right below the end of Lake Manapouri. Thus, its waters could fall down shafts and drive seven turbines that would generate electricity. The electric power would be transmitted to the national grid through the switchyard on the edge of the lake. (See diagram, page 17.) But how would all that water escape? The miners had to excavate a tailrace tunnel with a diameter of 30 feet [9 m] that would extend some six miles [10 km] under the mountains. This would allow the water to exit into Deep Cove in Doubtful Sound, one of New Zealand’s superb fiords. That tunnel alone required the removal of one million cubic yards [760,000 cu m] of rock.
Imagine the vast amount of rock that would have to be taken out of the mountain just for the water shafts and the turbine chamber. This chamber, or machine hall, alone is 364 feet [111 m] long, 128 feet [39 m] high, and 59 feet [18 m] wide. In length an American football field or a soccer field would fit into it. But first a tunnel was needed in order to reach and excavate the machine hall, where the turbines and generators were to be located. That was a unique challenge of its own!
This road tunnel, over a mile [2 km] long and with a steady gradient of one foot [1 m] every ten feet [10 m], spirals down to the machine hall. As we entered the mountain in our tourist bus, it was a sobering thought that we were descending into its very bowels.
When we finally got out of the bus and entered the turbine hall, it was like something out of science fiction—a vast science cathedral in the depths of a mountain! But one question puzzled me, How did they get all the heavy machinery onto the site for this complex project? The only points of access were either by sea or by lake. There were no roads. It was determined that it would be easier to bring in most of the generating machinery by sea. That left a mountain range barring the way to the power-station site. The solution? Build a road.
New Zealand’s Steepest State Highway
Work began in 1963 on the road link from Deep Cove to West Arm, “one of the most difficult roading ventures in the world,” according to one source. Why was that? “Rain, snow, rivers of mud and masses of tangled vegetation stretched the completion time from 12 to 24 months.” Some 14 miles [23 km] long, it finally cost NZ$4 per inch [cm]—a very expensive road! With gradients as great as 1 in 5, it became New Zealand’s steepest highway. Yet, it was the vital link for the movement of 87,000 tons of material from sea level, over the Wilmot Pass (2,200 ft [670 m]) to lake level. One load alone weighed 290 tons and required a 140-wheel transporter that was pulled by a bulldozer and grader and was pushed by yet another bulldozer! But the job was done.
Effects on the Ecology
How does this massive project affect the local ecology? Since most of the power station is underground, its visibility is minimal except for the switchyard and the transmission lines crossing the mountains. In the vastness of the region, even the power pylons and cables are dwarfed. But there is another question to answer.
If Lake Manapouri is, in effect, being emptied from the rear, how is its level maintained? One basic factor is the high rate of annual rainfall in the region. Manapouri township gets an annual average of 49 inches [1,250 mm], while the power station at West Arm gets 148 inches [3,750 mm]. Also, strict guidelines are followed in controlling the level of the lake so that it remains as close as possible to its natural mark. Since Lake Manapouri is at the upper end of a water system that involves Lake Te Anau and the Upper and Lower Waiau rivers, control weirs are used to maintain the level needed for the power station. When there is too much water for the generators to handle, the weir gates are opened to release the excess.
Who Benefits?
The installation of New Zealand’s largest hydropower station has been an example of international cooperation. The turbines were manufactured in Scotland, the generators in Germany, and the transformers in Italy. The first of the generators was commissioned in 1969. By September 1971 all seven were in operation. Who get the benefit of all this generated power? Most of it goes to the smelting works at Tiwai Point, and the rest goes into New Zealand’s national grid. The operation of the Manapouri power station and the continuity of the supply of electricity is critical to the smelting operation. Supply disruptions of longer than two hours can result in a shutdown for several months. The Manapouri power station and the smelting people therefore cooperate to ensure stability.
We took the bus ride over Wilmot Pass and down to Doubtful Sound. There we saw the tailrace waters from the Manapouri power station pouring into the silent sound. This sound, or fiord, has a curious feature. “The surface of the fiord is a fresh water layer sitting on top of denser sea water. In the fiord the fresh water remains a distinct layer—a river gently flowing over this enclosed sea.”—Manapouri to Doubtful Sound, by Barry Brailsford and Derek Mitchell.
Another powerboat took us for a serene ride along the sound. At one point the captain turned off the engines, and we listened to the sublime silence of that virgin paradise. An occasional birdcall echoed across the water. What a contrast to the raging power of the Manapouri hydroelectric station just a few miles away, hidden in the heart of a mountain.—Contributed.
[Diagram/Pictures on page 17]
(For fully formatted text, see publication)
Power-plant layout
Lake Manapouri
Lift shaft
Intake-gate shaft
Intakes and screens
Tailrace tunnel to Deep Cove
Emergency exit
Outdoor station
Power-cable shaft
Access tunnel
Transformer vault
Machine hall
Service tunnel
[Pictures]
Switchyard
Machine hall
Manapouri power station
[Picture on page 15]
Doubtful Sound, New Zealand
[Picture on page 16]
The access tunnel into the mountain and down to the machine hall